Engine knocking, or pre-detonation, can occur in spark ignition gasoline internal combustion engines. Knocking is caused by an undesirably rapid combustion in the combustion chamber, which results in shock waves. These shock waves can be heard by a vehicle driver and can, if persistent, lead to engine damage. There is a strong relationship between the ignition or spark angle and the occurrence of engine knocking.
It is generally desirable to operate an internal combustion engine with an advanced spark angle, that is, with an ignition spark that is advanced in time of a top dead center position for a cylinder, as this improves engine performance and fuel economy. If the spark angle becomes too far advanced, then engine knocking will occur. Various knock detection systems have therefore been proposed to allow an engine to be operated with a spark angle close to the point at which knocking will occur, while at the same time suppressing persistent knocking.
In one prior art system, disclosed in U.S. Pat. No. 5,485,380, a signal from a piezoelectric knock sensor is used to determine an average engine noise level over a range of engine angles when no combustion is taking place, and then to compare this with output from the same sensor when knocking may take place. If the output when combustion is taking place exceeds a certain level, then knocking is deemed to have taken place.
An advantage of measuring engine knocking noise and background engine noise in different engine angle ranges is that the ratio or comparison of the sensor signals is self-compensating with respect to the general level of engine noise or changes in sensor output due to ageing of the sensor.
It has been found, however, with lower capacity (e.g. 1.0 to 2.0 liter) engines, that it is more difficult to detect knocking than with larger capacity engines having more cylinders. The problem is particularly acute with engines that are fuel efficient and inherently noisier, for example, those having a relatively high compression ratio, lightweight pistons, and a faster fuel burn rate.